In order to improve performance of digital communication, multiple input multiple output (MIMO) systems are used to replace single input single output systems. In a MIMO system, both the transmitter and the receiver use multiple antennas. Multiple data streams or signals can be transmitted simultaneously by the transmitting antennas. Compared to a single input and single output system, the MIMO system can provide high data throughput by sending independent data streams simultaneously or provide more reliable communications by transmitting the same data stream by multiple antennas.
Orthogonal frequency division multiplexing (OFDM) technique is combined with MIMO to further improve the performance of digital communication. OFDM is a modulation scheme, in which a single data stream is split across several separate narrow-band channels at different frequencies. That means the available frequency band is divided into several subcarriers of smaller bandwidth. By using OFDM in a MIMO system, a frequency-selective MIMO channel is converted into several parallel frequency-flat MIMO channels. The interference among the channels with close frequency can be reduced so that to improve the accuracy of the received signals. Therefore, the combination of multiple input multiple output and orthogonal frequency division multiplexing can be used to improve the data throughput, reliability and sensitivity of wireless communication systems.
FIG. 1 illustrates an exemplary diagram of a multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) system. Transmitter 101 consists of m transmitting units represented by Tx1 to Txm to send m coded source signals S1 to Sm. Each transmitting unit is coupled to an individual antenna 101a. Each transmitting unit includes necessary circuits (e.g. Power Amplifier) to provide a signal to the antenna suited for radiation from the antenna. The channels between transmitter 101 and receiver 103 are illustrated by a set of dash arrows 102. The direction of each dash arrow indicates a data transmission direction in a channel between a transmitting antenna and a receiving antenna. Receiver 103 is equipped with n receiving units, illustrated by Rx1 to Rxn, to detect the transmitted signals. Each receiving unit is coupled to an individual receiving antenna 103a to receive transmitted signals. Each receiving unit includes necessary circuits (e.g. Low Noise Amplifier) to provide received signals for further processing. The received signals are represented by R1 to Rn, respectively. In MIMO processing unit 104, the demodulated signals Ŝ1 to Ŝm are extracted from the received signals based on the channel information between receiver 103 and transmitter 101.
In the MIMO-OFDM system, the channel information is a key factor of signal separation and demodulation. The demodulation performance of the MIMO-OFDM system is very sensitive to the channel information determined before decoding. In traditional methods, the channel information is usually estimated based on a training sequence in the preamble. However, the channel characteristics are constantly changing in a mobile communication system, the update based on the training sequences in the preambles, which are transmitted only periodically in some time slots, will not be dynamic enough to provide fast channel update. The estimation error between the estimated channel information and the actual channel information increases with the channel variation. This can cause serious performance degradation of the MIMO-OFDM system. By simply increase the occurrence of the training preamble may improve the accuracy of channel estimation. Nevertheless, the additional time slot for the preambles will take away valuable bandwidth for transmitted data.
In order to improve the performance of MIMO-OFDM systems, it is desirable to develop a channel estimation algorithm to update the channel information more frequently without sacrificing the valuable channel bandwidth or to provide more accurate control on the estimation error.